Known methods for treating wastewater containing an organic matter include the activated sludge method utilizing the aerobic respiration of microorganisms (e.g., Patent Document No. 1), and the anaerobic treatment method utilizing the anaerobic respiration of microorganisms (e.g., Patent Document No. 2).
In the activated sludge method, a sludge containing microorganisms (activated sludge) and wastewater are mixed together in a biological reactor tank (aeration tank), and the mixture is stirred while sending, into the biological reactor tank, an air needed for microorganisms to oxidatively decompose the organic matter in the wastewater. Thus, the organic matter in the wastewater is subjected to an oxidative decomposition treatment. However, the activated sludge method has a problem in that the aeration of the biological reactor tank requires a very large amount of electric power, and a large amount of sludge (dead microorganisms), which is an industrial waste, is generated as a result of the microorganisms actively metabolizing while breathing oxygen.
In contrast, in the anaerobic treatment method, since there is no need for aeration, the amount of electric power needed is significantly reduced from that of the activated sludge method. Also, the microorganisms can acquire only a small amount of free energy, thereby reducing the amount of sludge generation. However, as a product of anaerobic respiration, there is generated a biogas such as a methane gas, which is flammable and has a peculiar odor.